Efficient Electroluminescence from Type I Quantum Confined Structure Utilizing Organic Dye Materials
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ABSTRACT Improvement of electroluminescence (EL) efficiency by utilizing quantum confined structure, so-called type I superlattice, has been discussed. Superlattice structures, which consist of 8-hydroxyquinoline aluminum (Alq 3) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), and of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and Alq 3, and of (1,10-phenanthroline)-tris -(4,4,4-trifluoro -1-(2-thienyl)-butane-1,3-dionate) europium (III) (Eu(TTA )3phen) and (N,N'-disalicylidene-1,6-hexanediaminate) zinc (II) (1AZM -Hex) are studied. As a result, strong EL emission is observed in the type I superlattice structures. The mechanism of enhancement is discussed using energy band structure.
INTRODUCTION In order to realize commercial base of organic electroluminescent (EL) devices, fabrication of EL devices with high intensity and high efficiency is one of the key issues in their device performances. In this paper, improvement of EL efficiency utilizing quantum confined structure of 8-hydroxyquinoline aluminum (Alq 3) [1, 2] and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [3], and of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) and Alq 3, and of (1,10-phenanthroline)-tris -(4,4,4-trifluoro -1-(2-thienyl)-butane-1,3-dionate) europium (III) (Eu(TTA )3phen) and (N,N'-disalicylidene-1,6-hexanediaminate) zinc (II) (1AZM -Hex) [4] are discussed. Molecular structures of these materials used in this experiment are shown in Fig. 1.
Figure 1 Molecular structure of materials used in this study.
An energy band diagram, so called Type I superlattice in which electrons and holes are confined simultaneously in one material, is discussed for organic EL devices. Energy band diagrams are shown in Fig. 2, which are discussed in this study. Energy band of type I and type I' superlattice are shown in Figs. 2(a) and (b), and those of Alq 3/PBD and DCM/Alq 3, which belong to the Type I superlattice, are shown in Figs. 2(c) and 2(d), respectively.
Figure 2 Energy band diagrams of Type I and Type I' superlattices and those of Alq 3/PBD and DCM/Alq 3 systems.
EXPERIMENTAL Figure 3 shows a schematic of organic EL devices utilizing (a) Alq 3/PBD and (b) DCM/ Alq 3 superlattice structures, used in this study. The devices are fabricated using the OMBD system, and the devices consist of an indium-tin-oxide (ITO)-coated glass substrate, N, N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) hole transporting layer (40 nm), superlattice structure of emissive layer (40 nm) and a magnesium containing silver (Mg:Ag) cathode electrode. EL device whose emissive layer consists of Alq 3 containing DCM was fabricated for comparison. Organic thin films were fabricated using the organic molecular beam deposition (OMBD) system. The deposition was carried out at a background pressure of under 10-5 Pa. Organic materials were loaded into separate Knudsen cells (K-cells) and subsequently heated to their sublimation temperature, and then deposited onto the su
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